Review for British English Dictionary by Jordan McClements
Rated 5 out of 5 stars
Re the spell check as you type not working with version 24 of Thunderbird. I also had the same problem, but this was fixed (don't ask me why) by downloading and installing the US English version of the dictionary (but not actually using it)...
By the way everyone - it doesn't cost you anything to be polite.
To create your own collections, you must have a Mozilla Add-ons account.